1. Field of the Invention
The present invention relates to an image forming technology.
2. Description of the Related Art
It is important to stabilize the quality of an image formed in image forming devices. Generally, in electrophotographic image forming apparatus, image forming density (e.g., the amount of color material) becomes unstable due to variations in each part (e.g., amount of electric charge retained in color material) and variations in installation environment (e.g., temperature and humidity). The image forming density also becomes unstable by variations in sensitivity of photosensitive material and environmental variations in transfer member.
As an approach for stabilizing formed images, a method of controlling developing conditions (Japanese Patent Application Laid-Open No. 09-319270) or a method of modifying image data (Japanese Patent Application Laid-Open No. 2003-228201) are generally used.
In the method of controlling developing conditions, first, a patch image is formed on a photosensitive material or transfer member as an image conveyer. Next, the toner density of the formed patch image is detected. Depending on the detected toner density, the ratio of magnetic powder to toner in a developing device is controlled.
Similarly, in the method of modifying image data, the toner density of a formed patch image is detected. Depending on the detected toner density, the values of a γLUT (Gamma Look-Up Table) is modified. The γLUT is a table for performing one-dimensional transformation of image data. This γLUT can determine a value corresponding to the input data (primarily 0-255) and outputs the determined value (also 0-255).
However, the method of controlling developing conditions has a difficulty, since control response is typically low when developing conditions is changed. That is, there is a drawback in that it takes relatively longer time to settle the variation.
As compared to the method of controlling developing conditions, the method of transforming image data is advantageous with respect to the response of control, since feedback from the detection result of the patch image is applied to the γLUT. In this method, the transformation process of the image data using the γLUT is first performed, and then halftone process must be performed.
However, in recent years, it is becoming popular that halftone-processed image data is inputted to image forming apparatus. The halftone-processed image data as represented by 1 Bit Tiff has binary (i.e., 0 and 255) data only. Therefore, even if these data are processed using the γLUT, 0 and 255 are merely transformed to 0 and 255, respectively. As a result, when halftone-processed image is inputted, the method of modifying the γLUT using the patch image no longer makes sense.